The onset of mastocytosis occurs between birth and 2 years of age in approximately 55% of all cases; an additional 10% develop the disease before the age of 15 years. Mastocytosis in these age groups differs in many respects from mastocytosis that has its onset in adulthood. The typical presentation of pediatric-onset mastocytosis consists of cutaneous manifestations: either a solitary mastocytoma, urticaria pigmentosa, or, less commonly, diffuse cutaneous mastocytosis. Particularly in infants, bullous eruptions may occur. Mastocytosis in infants and children may involve internal organs, including the bone marrow and the gastrointestinal tract, although such manifestations appear to be less common in children than in adults. Plasma histamine levels may be elevated in pediatric-onset mastocytosis. Treatment usually involves the use of H1 and H2 antihistamines to control itching and to control the hypersecretion of gastric acid that may occur. The prognosis for children with mast cell disease is variable; approximately half of the children with urticaria pigmentosa may experience resolution of lesions and symptoms by adolescence.